This invention relates to magnetic tape reading equipment and more particularly to an electronics circuit for generating an electronic window within which a voltage transition, which constitutes a phase encoded data bit, may be detected,
Phase encoded data is recorded on a multi- track magnetic tape such that a one bit will be represented by a falling voltage transition and a zero bit will be represented by a rising voltage transition. These data bits are equally spaced along each track of a multi-track tape so that each data bit can be used not only for its data information but also for timing purposes. In this way, no separate clocking information need be recorded on the magnetic tape. Thus, a nine track tape can provide the data and timing for reading an eight bit byte plus a parity bit in parallel. If necessary, a phasing bit is used between like data bits to restore the voltage output to the phase required for the next data bit. For instance, if two consecutive one bits are to be recorded, and a one bit is a falling transition, then a phasing bit returning the voltage from a low level to a high level will be necessary between one bits. Likewise, a phasing bit comprising a falling transition is required between two zero bits, each of which is represented by a rising transition.
A TYPICAL MAGNETIC TAPE SYSTEM WILL HAVE 1,600 BITS PER INCH RECORDED ON EACH TAPE TRACK, WILL CONTAIN NINE TRACKS PER TAPE, AND WILL BE OPERATED AT TAPE SPEEDS OF FROM 45 TO 150 INCHES PER SECOND. Theoretically, if the magnetic heads were perfectly aligned and if the magnetic tape were perfectly recorded and guided past the read/write heads, then all of the bits in each byte could read out simultaneously. However, a variety of misalignment of "skew" conditions arise.
Mechanical skew is created either when each of the nine read/write heads is not perfectly in line with all other heads or if the alignment of the resultant head assembly is not absolutely perpendicular to the direction of tape movement. Skew can also result from any wobble of the magnetic tape that results in less than perfect instantaneous alignment between the magnetic tape and the head assembly.
Electrical skew may be created during the recording process when the individual bits in each byte, for a variety of electrical and mechanical reasons, are not recorded in line on the magnetic tape. Also, some electrical noise may be generated during the read process. These problems are aggravated at high tape speeds and where there are a large number of bits per inch on the magnetic tape.
Another problem associated with reading phase encoded data is that the electrical equipment must be able to distinguish between a data bit and a phasing bit. This is usually accomplished by generating an electrical timing window which opens slightly ahead of the time when a data bit is expected and which closes some time after the data bit has been received, thus inhibiting the reading of a phasing bit. These window generating circuits rely on the timing of the data bits to generate an appropriate window. Since the mechanical and electrical skew problems associated with the reading and writing of phase encoded data results in long and short term variations between data pulses, the window must be automatically adjustable to match these long and short term variations.
Prior window generating circuits typically comprise a phase lock loop circuit implemented from capacitive, resistive, and inductive components to generate a sequence of windows, and rely on some kind of feedback from the timing of the data bits received to compensate for long and short term variations between a nominal window rate and the rate at which the data is actually being received.
However, there are a variety of problems associated with window generating circuits implemented from discrete components. To begin with, a circuit that relies on circuit time constants must be tuned for its particular application. This tuning must compensate not only for the requirements of the particular system, tape speed and bits per inch; but also must be tuned to account for the tolerances of the components from which this particular circuit was produced. Thus, each circuit must be individually tuned during the manufacturing process. This can be a substantial undertaking if the circuit has several different modes of operation. There will typically be one mode of operation when the tape reader first starts to read a data transmission from the magnetic tape, in which case the circuit time constants will be rapidly adjusted to quickly lock in on the appropriate frequency of data reception. After some preliminary period the time constants will be adjusted more slowly so that a correspondingly longer amount of time will elapse before making small corrections to compensate for long term drifts in the bit rate. Thus, in each analog circuit there may be a variety of circuit elements that need to be individually tuned, adversely affecting the manufacturability of the circuit.
An analog circuit is also sensitive to electrical noise and temperature variations which the designer must take into consideration. Also, circuit parameters tend to drift with time and must be periodically realigned. Thus, analog circuits designed to read phase encoded data are difficult to manufacture and to maintain.
Analog circuits are also difficult to modify. If the equipment must be redesigned either initially to correct design defects or ultimately to allow the tape reading system to handle higher speeds or bit packing densities, these analog circuits must be either substantially or completely redesigned.
Thus, there exists a need for a magnetic tape reading circuit which is easily designed, maintained and modified, and which will operate reliably under a variety of adverse electrical, mechanical and thermal conditions.